Metallic coating of steel strips by hot dipping is usually performed by a process comprising essentially the following steps:
annealing of a running steel strip in a furnace under an inert or reducing atmosphere, in order to avoid an oxidation of the strip surface;
dipping of the running strip into a vessel containing a bath of metal or of a metal alloy in the liquid state; so the strip gets coated with metal/metal alloy as it exits the bath.
after the exit of the strip from the liquid bath, wiping of the metal/metal alloy layer by projecting a gas onto its surface, in order to ensure that the layer has an even and regular thickness.
The heating of the strip during its annealing step, before it enters the metal bath (in the following parts of the specification, it must be understood that when speaking of the “metal bath” or “metal layer”, any metal alloy baths and corresponding metal alloy layers such as Al/Al alloy or Zn/Zn alloy will also be encompassed by this expression) usually takes place in a direct-fired annealing furnace or a radiant tube annealing furnace. However, the use of these furnaces to heat the sheet can lead to the formation of oxides on the surfaces of the sheet, which must then be eliminated by additional pickling and/or shot blasting steps before coating. If not, the wettability of the liquid metal on the steel sheet surface is insufficient, inducing notably bare spots on the steel surface.
This drawback is particularly met when the strip composition includes significant amounts of easily oxidized elements like Si, Mn, Al, Cr, B, P and so on.
One can consider that the contents over which this drawback can be met are about 0.5% in weight for Si, Mn, Al, P and Cr, and 5 ppm for B, if these elements are taken isolately. But these limits can be sensibly lower when several of these elements are present in the steel. For example, an interstitial-free bake-hardenable steel with 0.2% of Mn, 0.02% of Si and 5 ppm of B may already undergo such wetting problems, due to the presence of B which rapidly diffuses up to the strip surface and makes the Mn and Si oxide precipitate as continuous films, leading to a bad wetting, rather than as nodules which would not be too detrimental to the wetting properties.
Generally speaking, this risk of bad wetting by liquid metal is also met on all high strength steels, since they comprise at least one of said elements, like dual-phase steels, TRIP (TRansformation Induced Plasticity) steels, TWIP (TWining-Induced Plasticity) steels, electric steels and so on. For dual phase steels, the amount of Mn is generally lower than 3% by weight, with addition of Cr, Si or Al in an amount generally lower than 1% by weight. For TRIP steels, the Mn amount is generally lower than 2% by weight associated with maximum 2% by weight Si or Al. For TWIP steel, the Mn amount can be as high as 25% by weight, associated with Al or Si (max 3% by weight).
Steels with a low density containing notably Al and/or Si in big amounts (up to 10% by weight) are also sensitive to this phenomenon, as well as, for example, high Cr stainless steels for thermal treatments.